1. Field of the Invention
This invention relates to a process for producing a diene polymer having markedly improved tensile strength and wear resistance and further having improved heat build-up, rebound resilience, processability and wet skid resistance.
Moreover, this invention relates to a rubber composition having improved tensile strength, wear resistance, etc., and more specifically to a rubber composition containing a terminal diene polymer having a high content of a trans-1,4 bond of a conjugated diene moiety.
2. Description of the Prior Art
Heretofore, diene rubbers have been used as a main material in large-sized tires of trucks, buses, aircraft, etc. Among the diene rubbers, natural rubbers are effective for improving heat build-up and tensile strength, styrene-butadiene copolymer rubbers for improving wear resistance and crack resistance, and polybutadiene rubbers having a high cis content for improving wear resistance, respectively. Rubber compositions containing suitable amounts of two or more of these diene rubber materials have been so far used according to improving purposes. For instance, regarding a rubber in a tread portion of a large-sized tire, a rubber composition comprising a natural rubber as a main component and blended therewith, a polybutadiene rubber having a high cis content has been generally used because a load exerted on the tread portion is quite high.
By the way, wear resistance and crack resistance, important properties of a large-sized tire, are contradictory properties. Accordingly, in the composition comprising the natural rubber and the polybutadiene rubber having the high cis content, when the amount of the polybutadiene rubber is increased, wear resistance is improved, but crack resistance is decreased; when the amount of the polybutadiene rubber is decreased, improvement in wear resistance becomes insufficient.
For this reason, many studies have been so far made about improvement of polybutadiene rubbers. For example, there have been proposed a method using a polybutadiene rubber having a wide molecular weight distribution to improve tackiness and green strength (Japanese Laid-open Patent Application (Kokai) No. 45337/1984) and a method using a polybutadiene rubber having a high molecular weight to improve wear resistance. Nevertheless, these methods have a problem that wear resistance is improved but processability is decreased.
A rubber composition containing a polybutadiene rubber having a high trans content has been proposed (Japanese Laid-open Patent Application (Kokai) No. 197749/1985). On this occasion, a polybutadiene rubber having a high trans content is used chiefly to improve processability and cold flow property of the rubber composition, and there is almost no effect of improvement in wear resistance.
Polybutadienes having a trans content of higher than 90% have been studied in Japanese Laid-open Patent Application (Kokai) Nos. 163951/1986, 168636/1986, 168637/1986 and 168639/1986. However, they are all concerned with resin materials, not with rubber compositions.
Next, upon looking at the situation of automobile tires, polybutadiene rubbers having a low vinyl content or styrene-butadiene copolymer rubbers having a low styrene content or a low vinyl content have been hitherto used to improve wear resistance. Nevertheless, the rubber compositions containing these rubbers as a main component have suffered a defect that wet skid resistance, a property required of tire treads, is decreased.
When a rubber composition containing as a main component a polybutadiene rubber having a high vinyl content or a styrene-butadiene copolymer rubber having a high styrene content is used to improve wet skid resistance, wear resistance is decreased; especially in case of a styrene-butadiene copolymer rubber having a high styrene content, rebound resilience is decreased and rolling resistance of a tire is increased.
The present inventors have found before that rebound resilience is remarkably improved by reacting a diene copolymer having active alkali metal and/or alkaline earth metal ends with a compound having in a molecule an atomic group of the formula --C(.dbd.M)--N&lt; (wherein M denotes an oxygen atom or a sulfur atom) to introduce the specific atomic group into the polymer (U.S. Pat. No. 4,647,625). The diene rubber polymer with the atomic group introduced is improved in not only rebound resilience but also wear resistance and wet skid resistance. However, in relation to environmental problems in recent years, low fuel consumption has been increasingly demanded, and levels of improvements in light weight and wear resistance of tires have become quite high accordingly. Nonetheless, the conventional methods can hardly meet such high levels.
A rubber composition containing a butadiene rubber having a high trans content of a butadiene moiety or a styrene-butadiene copolymer rubber has been proposed (Japanese Laid-open Patent Application (Kokai) No. 100146/1982). This proposal aims chiefly to improve processability or cold flow property or to improve decrease in wet skid resistance provided by using the styrene-butadiene copolymer rubber, and it has actually little effect of improvement in wear resistance.